A typical printer includes one or more print heads for applying ink onto paper. A typical print head includes a set of nozzles and a firing mechanism for ejecting ink drops through the nozzles. Examples of firing mechanisms include piezo-electric crystals that squeeze out ink drops through the nozzles and heating elements that boil out ink drops through the nozzles.
It is often desirable to provide a printer with an ink drop detector. An ink drop detector may be used to detect whether ink drops are being ejected from individual nozzles of a print head. For example, an ink drop detector may be used to determine whether nozzles are clogged and would benefit from cleaning or whether individual nozzles have failed permanently.
One type of prior ink drop detector that may be employed in printers is an electrostatic drop detector. An electrostatic drop detector may include a conductive surface that functions as a charging element and a sensing element. A print head may be positioned to fire ink drops at the conductive surface. A high voltage may be applied to the conductive surface to create a relatively strong electric field that induces an electrical charge into the ink drops ejected from the print head. The charged ink drops that strike the conductive surface usually impart an electrical pulse into the conductive surface. Signal processing may be used to derive a drop detection indicator from the electrical pulses imparted by the charged ink drops onto the conductive surface.
The firing mechanism in a typical prior print head may create electrical noise that coincides with ink drop ejection. The electrical noise caused by firing pulses in a print head may be mistaken for charged ink drops by the electrostatic drop detector and give false indications of ink drop ejection.
In addition, the electrical noise caused by firing pulses in a print head may increase the signal-to-noise ratio which leads to more expensive detection circuitry and more complex signal processing.